Conventional grounded antenna towers have mounted on the top, for example, a directional antenna array for radiating (radiating and radiator being used to indicate the radiating and/or receiving of electromagnetic signals) signals at selected relatively high frequencies with wave lengths, for example, on the order of 20, 15 and 10 meters and V.H.F. A coaxial cable from the radio equipment extends along the tower and is connected to the directional array. Sometimes plural cables may be connected to respective directional arrays adapted to selected respective frequencies.
A grounded antenna tower also may be fed an electrical signal and used in conjunction with additional equipment as the radiator for signals having lower frequencies than those to which directional arrays are adapted. Such additional equipment includes a gamma section and impedance matching circuitry. Such equipment, as is known, can adapt a relatively short antenna tower on the order of, for example, 5 meters to about 30 meters in height, for use on the amateur radio bands of 40 meters, 80 meters, or 160 meters, with each having frequencies in ranges of about 7-7.3 MHz., 3.5-4 MHz., and 1.8-2 MHz., respectively. By using such a gamma section and impedance matching circuitry to shunt feed an antenna tower and to thus convert the tower itself to a radiator, even though the tower height may be relatively small, the versatility of the tower is increased.
To shunt feed a tower so as to use the same as a radiator in the 40, 80 or 160 meter wave length bands, a gamma rod or gamma matching section, which may be simply an electrically conductive rod or wire, is mounted in substantially parallel relation to the tower spaced apart a relatively small distance therefrom. The bottom end of the gamma section is connected by impedance matching circuitry to the bottom of the tower, which is grounded, and to a coaxial cable from the radio equipment, which is generally located remotely from the antenna. In the past, the gamma section has been about 4 or 5 meters in length and has had its upper end electrically tied or connected to the tower at a height along the latter that was specifically adjusted for tuning purposes to obtain a signal strength of adequate value.
The purpose of the impedance matching circuitry, such as conventional gamma matching or omega matching circuitry, was to provide about 120 to about 500 pf. capacitance to match the shunt fed radiator with the impedance of coaxial cable coupling the same to the radio equipment. Such coaxial cable typically has an impedance on the order of about 50 ohms. When shunt feeding an antenna tower, it is desirable that the standing wave ratio (SWR) be as close to 1 as possible. Conventional impedance matching circuitry generally can be adjusted with an SWR meter connected in series therewith to obtain such condition.
To the best knowledge of the applicant, in the past, the shunt feeding of an antenna tower was provided to enable the tower as an effective radiator only in a single one of the 40, 80, or 160 meter wave length bands. In one instance, though, the antenna tower was shunt fed with a single gamma section to enable the tower as a radiator in both the 40 and 80 meter bands, but in that instance it was necessary to adjust the height of the upper tie point of the gamma section to the tower in dependence on the particular wave length being utilized. Moreover, in another instance that a shunt fed tower was enabled as a radiator in more than one of the 40, 80 and 160 meter bands, a separate gamma section and impedance matching circuit was used for each respective band.
It would be desirable to be able to utilize a single shunt feed mechanism, i.e. with only a single gamma section and a single impedance matching circuit, to enable an antenna tower as effective radiator in plural wave length bands, such as, for example, the 40 and 80 meter bands or the 80 and 160 meter bands. Moreover, it would be desirable to be able to use a single shunt feed mechanism to enable a single antenna tower as an effective radiator for all three of those bands. Further, it would be desirable to accomplish the foregoing without having to adjust the upper tie point of the gamma section to the antenna tower.